In order to have a better understanding of the mechanism of Fv-1 restriction, studies on B-type cells infected with N-tropic MuLV and tropic MSV pseudotypes have been performed. Studies on N-type cells infected with B-tropic viruses have been carried out to determine whether the B-tropic MuLV is able to overcome Fv-1 restriction in the N-type cell system. At high multiplicities, this restriction is not overcome. This makes new biochemical approaches possible to determine the mechanism of Fv-1 restriction.